Selecting the sensitive solution

There has been significant progress in trenchless techniques in recent years. Tim Read outlines some of the benefits.

Trenchless technology is not a new technique in water mains and sewerage network maintenance. But the progress made in its application and processes in the past couple of years is considerable.

Previously limited in its uses, its versatility and capacity have been vastly improved by investment in research and development.

Open-trench work is still the best option for small-scale, emergency maintenance work, and for site work where surface disruption is not a critical factor. But, when the task involves concerns over disruption, and can be planned in advance, trenchless technology must be the first consideration.

May Gurney has been at the forefront of this investment. And, by tailoring our research to our customers' needs, it can have an immediate impact.

An example of this is our work with water mains rehabilitation. We identified a need to better address the challenges set by the requirement of water companies to renovate or replace asbestos cement (AC) water mains.

With one of our key clients, we set up a research and development programme to technically assess the suitability of a new high-build (HB) spray-lining system for use in refurbishing AC mains. The idea is that the process is tested to a level of confidence that it will result in its adoption as a standard trenchless practice.

Various trenchless processes have been used to renovate AC mains with varying success. Examples include on-line replacement by pipe-bursting.

This process offers advantages over the alternative open trenching, including minimising risk to operatives. But concern remains about the acceptable levels of environmental and occupational safety risks that remain. For example, fragments of AC pipe remain in the ground as a potential long-term liability, and a possible health risk to anyone excavating in these locations in the future. The main advantage of the HB lining process is that the bulk of the AC pipe remains undisturbed and therefore the environmental and health risks are significantly reduced.

Lack of disruptionThe most obvious benefit of trenchless engineering is the comparative lack of disruption. But, sometimes, open-trench work is not possible. This was the case when May Gurney undertook a contract that in the picturesque villages of Kenn and Kennford in the South-west of England. The task was to reduce infiltration on a 1,100m length of 150mm and 225mm diameter sewers.

Renewal of the sewers was the easy option by open-trench replacement. The new pipe would be relatively small and could be laid in shallow good ground. But the impact of trenching on the local community was considered unacceptable.

This was due to narrow roads, scarring of the highway, access problems for residents, disruption while importing and exporting backfill materials, and the risk of pollution to the adjacent River Kenn.

As well as being less invasive, a trenchless technique reduced risks to our operatives, eliminated or reduced risks to other utilities in the village such as the overhead electricity and telephone cables, and allowed for maximum productivity.

There were only two realistic trenchless options: localised patching across each joint, or cured-in-place pipe (CIPP) lining between manholes. A suitable CIPP lining was chosen to meet the engineering and communities' needs.

A public consultation was arranged at the local parish hall to inform and consult residents. This forum also gave us the rare opportunity to demonstrate directly the benefits of trenchless technology.

A project where there was potentially more at stake was May Gurney Utility Services and South West Water's involvement in the water mains rehabilitation programme in Devon and Cornwall.

Not only did this include work in the popular tourist destination of Marazion during the summer season but also included a 450m section of existing 150mm diameter cast iron pipe that ran straight underneath a Environmental Special Protection Area (SPA) known as the Long Green.

Sensitive locationThese SPAs are strictly protected sites, so this area was a highly sensitive environmental location. It is the largest remaining reed swamp in the South-west. And, as such, is an important and excellent example of an area of natural biodiversity.

Consultation with third-party stakeholders (not the client), timing of the works and trenchless technique selection was crucial to its success.

After careful consideration of trenchless options including pipe-bursting, directional drilling and slip-lining, Polymeric (PU) Lining of the water main (a fast-curing spray-lining technique developed over several years in conjunction with South West Water) was selected as the most suitable option for rehabilitating the water main through the area.

May Gurney Utility Services has been supporting its client South West Water to achieve its Drinking Water Inspectorate (DWI) Section 19 Water Quality undertaking in the South-west of England for several years. For the year 2005/6, May Gurney's contribution hit 365,000m (or 1% of the world's circumference).

The primary trenchless system employed in this programme is that of cleaning and PU spray lining the existing water mains to improve water quality to many consumers. The process therefore eliminates the need for complete replacement, without its associated high cost, social disruption and environmental damage.

It is the sheer volume and mixture of trenchless work with its attendant demand for robust planning, optimisation and safe site delivery that can be regarded as a major engineering and logistical achievement.

On this project with South West Water, for example, there was a planning resource in place including a design and planning manager and several dedicated planners. These planners were spread throughout the region assisting and developing the design packs that will form the agreed phases of trenchless work some 12 to 18 months ahead of the construction phase.

Currently, these trenchless entrepreneurs have already completed design packs for the current programme (2006-2007) and half of next year's programme.

This planning resource will be maintained to ensure that all planning is substantially completed ahead of any implementation on site to ensure the right trenchless solution has been found at the right cost and social benefit.

May Gurney has consistently offered South West Water and our other water industry clients throughout the UK value for money through optimisation of the best and most technically appropriate trenchless engineering solution for any given scheme.

Trenchless technology, while not the solution in every case, is increasingly the preferred option. It requires a deep understanding of the situation on site and the options available, linked to a robust process for planning and execution. And a commitment to public consultation and information maximises the awareness of the project, and its success.